1. Field of the Invention
The present invention relates to a technology for conducting an electric/magnetic field analysis using a finite difference time domain method for dividing an analysis space by a polyhedron cell and differentiating a spatial domain.
2. Description of the Related Art
For one of methods for conducting an electric/magnetic field analysis on antennas, electronic equipment and the like, a finite difference time domain (FDTD) method is used. The FDTD method is a finite difference method proposed by K. S. Yee. Since the FDTD method has an advantage that the necessary amount of memory is proportional to the number of cells, over other methods, attention is paid to the FDTD method as a useful method in the electric/magnetic field analysis.
In the FDTD method, a spatial domain is differentiated by dividing an analysis space by a polyhedron called a cell. For the polyhedron, usually a hexahedron, that is, a cube or a rectangular parallelepiped, is adopted. In the cell, generally magnetic field intensity and electric field intensity are disposed at the center of each surface and at the center of each side constituting the surface. By spatially shifting both magnetic and electric field intensity by half a cell and differentiating them, Maxwell's equations that the rotation of an electric field generates a magnetic field and the rotation of a magnetic field generates an electric field can be directly solved. The size of a cell is generally set to 1/10 or less of the shortest target wavelength.
Since an analysis space is divided by such a cell, a step approximation method for approximating its shape (boundary) step by step along the side (grating) or surface is conventionally used to express an analysis target object, such as a conductor and the like. However, in the step approximation method, in reality, there is often a great difference between an expressed shape and an actual shape depending on its cell size. For example, as shown in FIG. 1, if an analysis target object is a circular patch antenna 11, the boundary of the section is expressed as shown in FIG. 2 when its cell size is fairly large. Accordingly, the difference between the expressed shape and its actual shape becomes large.
The great difference in the shape of a boundary degrades analysis accuracy. Therefore, in order to obtain sufficient analysis accuracy, its cell size must be set small. However, if the cell size is set small, the number of cells increases. As the number increases, calculation cost, that is, necessary computer sources and calculation time increase. Therefore, in order to suppress the calculation cost while realizing sufficient analysis accuracy, it is important to express the shape of an analysis target object with high accuracy.
For the method for expressing the shape of an analysis target object with high accuracy while suppressing the increase of the number of cells, a method for transforming the shape of a cell can be used. However, the method requires modeling according to the shape of the analysis target object, which lacks generality. Therefore, in order to express the shape of an analysis target object with high accuracy, it is important to not only to suppress the increase of the number of cells, but also to give generality.